1. Field of the Invention
The present invention relates to a surface light source device of side light type and a LCD (liquid crystal display) with backlighting arrangement to which the surface light source device is applied. In particular, the present invention relates to improvement of the surface light source device employing guide plate provided with two incidence end faces, the improvement providing an increased emission toward a frontal direction (direction of a normal with respect to an emission face of the guide plate) and corresponding providing an improved screen luminance of the LCD.
2. Related Art
Surface light source devices of side light type are well known as devices for outputting illumination flux with large cross section, being applied to illumination such as backlighting of LCD.
A surface light source devices of side light type disposed for backlighting of LCD supplies a LCD panel with illumination light from behind the LCD panel. In general, a surface light source devices of side light type comprises a guide plate and a primary light source. The guide plate has major faces, one providing an emission face and the other providing a back face. The primary light source is disposed besides an incidence end face (a minor face) of the guide plate to supply the guide plate with primary light through the incidence end face.
This arrangement is suitable for thin-shaped overall structure. The primary light source is usually a rod-shaped light source such as cold cathode lamp. Point-like light sources such as LED (Light Emitting Diode) are employed in some cases. Illumination light emitted from the primary light source is introduced into the guide plate through the incidence end face. Thus introduced illumination light propagates within the guide plate, providing illumination output from the emission face of the guide plate on the way of propagation.
A typical and known guide plate is uniform in thickness overall and another typical and known guide plate has wedge-shaped cross section. According an prior art applied to the former, primary light is supplied through two end faces (i. e. minor faces providing incidence end faces) opposing to each other (i. e. oppositely located to each other).
Primary light is supplied by, for instance, rod-like cold cathode lamps which are disposed beside the incidence end faces, respectively. This will give greater illumination output compared with a surface light source device of side light type which is supplied with primary light through only one incidence end face. In some cases, an emission face or a back face of the guide provides a rough surface or a micro-lens surface to promote emission from the guide plate. Such a surface is called xe2x80x9cemission promoting facexe2x80x9d.
Surface light source devices of side light type with strong illumination output are suitable for backlighting of LCD applied to, for instance, car navigation system. This is because bright display will enable information indication to be instantaneously read.
The above-mentioned surface light source device supplied with primary light through both incidence end faces is not free from problem that emission to a frontal direction (direction of a normal with respect to the emission face) tends to be short, either.
An object of the present invention is to provide a surface light source device of side light type improved so that emission to a frontal direction of a guide plate (i. e. direction of a normal with respect to an emission face) increases and to provide a LCD provided with a backlight arrangement to which the surface light source device is applied.
The present invention is directed to an improvement of surface light source device of side light type which comprises a guide plate, having major faces to provide an emission face and a back face and being provided with a pair of incidence end faces located oppositely to each other across the guide plate, and a primary light source to supply primary light through the pair of incidence end faces.
According to the improvement, the back face is provided with a great number of fine projection rows. Each projection row has a pair of slopes running approximately at the right angle with respect to the incidence end faces. The emission face provides an emission promoting face to promote illumination light output. The emission promoting face may be a light scattering face which scatters illumination light at the emission face. Scattering power of the light scattering face preferably tends to be increasing toward a central portion from both incidence end faces of the guide plate.
It is also preferable that a light control member is disposed along an emission face of the guide plate employed in the surface light source device to correct directivity toward a frontal direction regarding in a plane perpendicular to the incidence end faces.
Furthermore, the present invention provides an improved liquid crystal display by arranging the improved surface light source device behind a LCD panel.
Increased emission to a direction of a normal is achieved by a great number of projection rows formed on the back face of the guide plate.
Since individual pair of slopes forming each projection row run approximately at right angle with respect to the incidence end faces, directivity correction is effected so that emission is gathered inward from right and left as viewed from an incidence end face. The emission face providing an emission promotion face scatters inner incidence light to promote emission. The emission promotion face may be, for instance, a matted face or a face to which light diffusive ink is deposited.
If scattering power of the light scattering face tends to be increasing toward a central portion from both incidence end faces of the guide plate, short of emission is prevented, which otherwise would tend to occur in the central portion distant from both incidence end faces.
Features of a surface light source device in accordance with the present invention is reflected onto image display quality of LCD with backlighting to which the surface light source device is applied, leading to an increased luminance as viewed from a frontal direction (direction of a normal) with respect to the display screen.
The above-described and the other features will be easily understood from detailed description of the embodiments with referring to the drawings.